Method for controlling an electrical machine and an inverter

ABSTRACT

A method for controlling an electrical machine while it is connected to an inverter comprising phase-specific switching components generating an output voltage, an optimum switching table arranged to select a switching combination for the switching components on the basis of the stator flux and torque of the electrical machine, the method comprising steps of defining the stator current vector (i s ) of the electrical machine, and determining the rotation speed of the electrical machine. The method also comprises the step of defining a switching combination for the switching components on the basis of the stator current vector (i s ) and stator current reference vector (ψ s, ref ) of the electrical machine when the determined rotation speed is lower than a predefined limit.

BACKGROUND OF THE INVENTION

The invention relates to a method for controlling an electrical machineand to an inverter, the electrical machine being connected to theinverter comprising phase-specific switching components generating anoutput voltage, an optimum switching table arranged to select aswitching combination for the switching components on the basis of thestator flux and torque of the electrical machine, the method comprisingsteps in which the stator current vector and the rotation speed of theelectrical machine are determined.

To reliably control electrical machines at a wide speed range without adirect feedback on the rotation speed or position angle causesconsiderable problems in the present control systems of machines.Especially when the electrical machine is a synchronous motor, thecontrol of the rotation speed in all speed ranges is difficult due tothe inaccuracy of the estimation of the stator flux which is the basefor many control systems. This problem is especially enhanced whenoperating at low rotation speeds. A rotating-field machine, such as asynchronous machine, can be started using modern control methods basedon direct torque control, but the load properties of the machine remaininadequate at low rotation speeds. In practice, it is, however, notpossible to use methods based on direct torque control continuously atzero speed and low rotation speeds without feedback on the rotorposition angle.

An error forms between the actual stator flux and the estimate formed ofit in drives based on direct torque control. In such drives, thedrifting of the stator flux is usually corrected by using a currentmodel made for the machine. The use of a current model cannot, however,prevent a static torque error. A rotating-field machine can becontrolled reliably at low rotation speeds by using stator currentcontrol in which the operation of the machine is controlled on the basisof the stator current.

BRIEF DESCRIPTION OF THE INVENTION

It is an object of the present invention to produce a method avoidingthe above-mentioned drawbacks and making it possible to reliably controlan electrical machine at a wide rotation speed range utilizing atransfer from one control method to another when the rotation speedchanges. This object is achieved by a method of the inventioncharacterized in that the method also comprises a step in which aswitching combination is defined for switching components on the basisof a stator current vector and a stator current reference vector of theelectrical machine when the determined rotation speed is lower than apredefined limit.

The method of the invention is based on the idea that at higher rotationspeeds the electrical machine is reliably controlled utilizing a controlmethod based on direct torque control, but at lower rotation speeds thecontrol method is changed to one based on stator current control. Thechange of control method provides a considerable advantage in that thedrive remains stable even at low speeds.

The invention also relates to an inverter comprising phase-specificswitching components generating an output voltage, an optimum switchingtable arranged to select a switching combination for the switchingcomponents on the basis of defining values, and a frequency definitionelement arranged to determine the rotation speed of the electricalmachine controlled by the inverter. The inverter of the invention ischaracterized in that the inverter also comprises a selection elementarranged to change the defining values used as the basis for theswitching combination for the switching components depending on therotation speed of the electrical machine.

The inverter of the invention is based on the idea that when therotation speed of the electrical machine exceeds a given limit value,the inverter changes the defining variables used as a basis formodulation.

By means of the inverter of the invention, the advantages provided bythe method of the invention can be realized with a simple structureusing the same modulator to generate output voltages both in a controlmethod based on direct torque control and in one based on currentcontrol.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in greater detail by means ofpreferred embodiments with reference to the attached drawing, in which

FIG. 1 shows a general presentation of the generation of an outputvoltage reference of an inverter according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows, on a general level, the generation of output voltagereferences of the inverter when utilizing an optimum switching table 1and blocks 2, 3, 4 forming the logic variables necessary for the optimumswitching table. The optimum switching table is a table containinginformation for all combinations of the incoming logic variables on howthe output voltages of the inverter should be generated. In FIG. 1, thelogic variables κ, φ, τ are formed of three blocks 2, 3, 4 receivingtheir individual inputs from the corresponding reference variablesκ_(ref), φ_(ref), τ_(ref). The first block 2 is a sector selector whichdefines the position angle of the vector variable acting as thereference variable κ_(ref) and produces the logic variable κ as itsoutput. The value of the variable κ thus changes as the angle of thereference variable moves from one sector to another. In the case of FIG.1, the sector selector contains six sectors and, correspondingly, thelogic variable κ can obtain six different values.

The logic variable φ affects the magnitude of the stator flu of theelectrical machine and the variable can obtain three different valuesdepicting the need to increase, decrease or maintain the presentmagnitude of the flux. Block 3 controlling the need to adjust themagnitude of the flux receives as input the reference variable φ_(ref)ref depicting the current value of the stator flux magnitude incomparison with the desired flux value. Correspondingly, the logicvariable τ can obtain three different values. The variable τ depicts theneed to increase, decrease or maintain the torque produced by theelectrical machine. Block 4 controlling the change in torque receives asinput the reference value τ_(ref) depicting the ratio of the torqueproduced by the electrical machine to the desired torque. The threeabove-mentioned logic variables κ, φ, τ are forwarded to the optimumswitching table which, on the basis of said variables, generates theoutput voltage of the inverter.

Direct torque control of an electrical machine is based on the use ofthe above-mentioned table while the reference variables are the statorflux vector ψ_(s)(κ_(ref)), the difference (φ_(ref)) between the squaresof the absolute values of the stator flux reference vector ψ_(s, ref)andthe stator flux vector ψ_(s), and the difference (τ_(ref)) between thetorque reference t_(e, ref)and the calculated torque of the electricalmachine. By using such reference variables, the electrical machine canbe controlled reliably and dynamically without direct feedback data onthe rotation speed or position angle of the machine. However, when theelectrical machine is a synchronous machine, problems arise in definingthe stator flux at low rotation speeds causing the load properties ofthe machine remain inadequate.

In the method of the invention, the switching combination for theswitching components is defined on the basis of the stator currentvector is and the stator current reference vector i_(s, ref) of theelectrical machine when the determined rotation speed is lower than apredefined limit. The same modulator formed by the optimum switchingtable is then used to generate the output voltage references S_(A),S_(B), S_(C) of the inverter in a control method based on statorcurrent.

The rotation speed and stator current vector of the electrical machineis determined according to the method of the invention. The rotationspeed can be determined without direct measurement from the motor shaftby means of the angular speed of the stator current vector, forinstance. According to the invention, the control basis is changedaccording to the rotation speed of the machine.

The implementation of the stator current control following the principleof direct stator flux and torque control is based on the fact that, atlow rotation speeds, the proportion of the electromotive force producedby the electrical machine is small as compared with the voltage dropsand the voltage of the intermediate circuit. In this case, it can beassumed that the direction of change of the stator current correspondsto the direction of the voltage vector being used. The transientinductance L′_(s) of a rotating-field machine having a smooth air gapcan be assumed to be independent of the rotor position, in which casethe voltage equation corresponding to a given voltage vector is${u_{s}\left( {S_{A},S_{B},S_{C}} \right)} \approx {L_{s}^{\prime}{\frac{\Delta \quad i_{s}}{\Delta \quad t}.}}$

With salient pole electrical machines, the equation is not quitecorrect, but when the switching frequency of the inverter is highenough, it can be assumed that the direction of change of the currentdefined by the equation is accurate enough for stator current control.The stator current control can then be implemented on the basis of theoptimum switching table using direct stator flux and torque control.

According to a preferred embodiment of the invention, the definition ofthe switching combination for switching components comprises steps inwhich the difference between the squares of the absolute values of thestator current reference vector i_(s, ref) and the defined statorcurrent vector i_(s) is calculated to obtain the reference amplitudeψ_(ref). This difference |i_(s, ref)|²⁻|i_(s)|² depicts the need forincreasing the stator current. Block 3 adjusting the magnitude of thestator current executes the clause sign|i_(s, ref) ²⁻i_(s) ²|, in whichcase the output of the block is the logic variable ψ which contains thesign of the calculated difference. Block 3 contains hysteresis, ifnecessary, whereby the value of the logic variable remains more constantwhen the reference amplitude is close to zero.

According to a preferred embodiment of the invention, the vector productbetween the stator current reference vector i_(s, ref) and the definedstator current vector is calculated to obtain the τ_(ref). Said vectorproduct i_(s, ref)×i_(s) depicts the angular difference between thestator current reference vector and the stator current vector. Thereference τ_(ref) obtained as the vector product is forwarded to block 4which executes the clause sign i_(s, ref)×i_(s)|, i.e. produces asoutput the sign of the vector product, i.e. the logic variable τ. Theoutput of block 4 is further connected to the optimum switching table 1.Like block 3 adjusting the magnitude of the stator current, also block 4comprises hysteresis, if necessary.

The definition of the switching combination for switching components,i.e. the output voltage vector, of the method of the invention comprisesa step in which the switching combination for the switching componentsis defined on the basis of the signs of the reference amplitude φ_(ref)and the reference τ_(ref) and the position angle of the stator currentvector i_(s). The position angle of the stator current vector is definedin sector selector block 2 which checks the position angle of therotating current vector. When the position angle changes, the currentvector moves from one sector to another. The sector selection blockproduces as output the logic variable κ which, in the case of theexample in FIG. 1, can obtain six different values. The value of thevariable κ indicates what the direction of the current vector is and, byusing the optimum switching table 1, the required output voltage vectorcan be selected on the basis of all three logic variables so as tocontrol the electrical machine in the desired manner.

The method of the invention is implemented at its simplest by utilizingthe inverter of the invention which comprises phase-specific switchingcomponents generating the output voltage, an optimum switching tablearranged to select a switching combination for the switching componentson the basis of defining values, and a frequency definition elementarranged to determine the rotation speed of the electrical machinecontrolled by the inverter. The inverter of the invention also comprisesa selection element 5 arranged to change the defining values used asbasis for the switching combination of the switching componentsdepending on the rotation speed of the electrical machine. In theexample in FIG. 1, the selection element 5 comprises three change-overswitches, by means of which the defining values used by the inverter arechanged. The change is performed depending on the frequency of theelectrical machine.

According to a preferred embodiment of the invention, the invertercomprises a current definition element arranged to define the statorcurrent vector i_(s) of the electrical machine, a flux definitionelement arranged to define the stator flux vector ψ_(s), of theelectrical machine, and a torque definition element arranged to definethe torque t_(e) of the electrical machine. In addition, according tothe preferred embodiment, the defining values used as basis for theswitching combination of the switching components comprise alternativelyeither the position angle of the stator flux vector ψ_(s), thedifference between the squares of the absolute values of the stator fluxreference vector ψ_(s, ref) and the stator flux vector ψ_(s), and thedifference between the reference torque t_(e, ref) and the torque t_(e)of the electrical machine, or the position angle of the stator currentvector i_(s), the difference between the squares of the absolute valuesof the stator current reference vector i_(s, ref) and the stator currentvector i_(s), and the vector product of the stator current referencevector i_(s, ref) and the stator current vector i_(s). The frequencyconverter thus selects the defining values it uses from the abovedefining values depending on the rotation speed of the electricalmachine.

According to a preferred embodiment of the invention, the inverterfurther comprises a selection element 6 arranged to select the referencestator flux to be used according to the frequency of the electricalmachine. In current control, i.e. when operating at low rotation speedsof the electrical machine, the magnitude of the reference stator fluxcan be set to differ from the value used in direct torque control. InFIG. 1, the reference stator flux selected with the selection element 6is forwarded to a ramping block 7 with which it is possible to ensure acontrolled and even transfer from one reference variable to anotherduring the change.

It is obvious to a person skilled in the art that the basic idea of theinvention can be implemented in many different ways. The invention andits embodiments are thus not restricted to the examples described above,but can vary within the scope of the claims.

What is claimed is:
 1. A method for controlling an electrical machinewhile it is connected to an inverter comprising phase-specific switchingcomponents generating an output voltage and an optimum switching tablearranged to select a switching combination for the switching componentson the basis of stator flux and torque of the electrical machine, themethod comprising steps of: defining a stator current vector (i_(s)) ofthe electrical machine, and defining a rotation speed of the electricalmachine, comprising defining a switching combination for the switchingcomponents on the basis of: the stator current vector (i_(s)) and astator current reference vector (i_(s, ref)) of the electrical machinewhen the determined rotation speed is lower than a predefined limit, andthe stator flux and torque of the electrical machine when the determinedrotation speed is higher than the predefined limit.
 2. A method asclaimed in claim 1, wherein the definition of the switching combinationfor the switching components of the inverter comprises the steps of:calculating a difference of squares of absolute values of the statorcurrent reference vector (i_(s, ref)) and the defined stator currentvector (i_(s)) to obtain a reference amplitude (φ_(ref)), calculating avector product between the stator current reference vector (i_(s, ref))and the defined stator current vector (i_(s)) to obtain a reference(τ_(ref)), and defining the switching combination for the switchingcomponents on the basis of signs of the reference amplitude (φ_(ref))and the reference (τ_(ref)) and position angle of the stator currentvector (i_(s)).
 3. An inverter comprising phase-specific switchingcomponents generating an output voltage, an optimum switching tablearranged to select a switching combination for the switching componentson the basis of defining values, and a frequency definition elementarranged to determine a rotation speed of an electrical machinecontrolled with the inverter, said inverter comprising: a selectionelement arranged to change the defining values used as basis for theswitching combination of the switching components depending on therotation speed of the electrical machine, the defining values being oneof: a stator current vector (i_(s)) and a stator current referencevector (ψ_(s, ref)) of the electrical machine when the determinedrotation speed is lower than a predetermined limit, and a stator fluxvector (ψ_(s)) and a torque (t_(e)) of the electrical machine when thedetermined rotation speed is higher than the predetermined limit.
 4. Aninverter as claimed in claim 3, comprising a current definition elementarranged to define the stator current vector (i_(s)) of the electricalmachine, a flux definition element arranged to define the stator fluxvector (ψ_(s)) of the electrical machine, and a torque definitionelement arranged to define the torque (t_(e)) of the electrical machine,wherein the defining values used as basis for the switching combinationof the switching components comprise alternatively either: a positionangle of the stator flux vector (ψ_(s)), the difference between squaresof absolute values of the stator flux reference vector (ψ_(s, ref)) andthe stator flux vector (ψ_(s)), and the difference between referencetorque (t_(e, ref)) and the torque (t_(e)) of the electrical machine, ora position angle of the stator current vector (i_(s)), the differencebetween squares of absolute values of a stator current reference vector(i_(s, ref)) and the stator current vector (i_(s)), and the vectorproduct of the stator current reference vector (i_(s, ref)) and thestator current vector (i_(s)).
 5. An inverter as claimed in claim 3,wherein the inverter further comprises a selection element arranged toselect a used reference stator flux according to the frequency of theelectrical machine.